1. Field of the Invention
The present invention relates to an optical wave guide, an optical component and an optical switch.
2. Description of the Background Art
In recent years, communications by use of optical fibers that enable the transmitting a large amount of data at high speed have become the mainstream in the art. Accordingly, optical wave guides for guiding optical signal to be used at connections and the like with optical fibers, light sources, light detecting elements require a more compact size and high precision with less transmitting loss.
Such optical wave guides are made into a structure wherein a portion whose refractive index is slightly higher is made on the surface of a substrate as a core, and they are manufactured through a semiconductor process, for example, as shown in FIG. 1.
The manufacturing process of an optical wave guide by semiconductor process is explained with reference to FIG. 1 below. First, a cladding material is accumulated and hardened on a substrate 1, and thereby, a lower cladding layer 2 is formed (FIG. 1A, FIG. 1B). Thereafter, a core material is accumulated and hardened on the lower cladding layer 2, and thereby, a core layer 3a is formed (FIG. 1C). Then, resist 4 is applied onto the surface of the core layer 3a, an exposure mask 5 is placed onto the resist 4, and exposure is made by ultraviolet ray radiation (FIG. 1D). After exposure, the resist 4 is developed, and thereby, patterning is made. Only a portion for forming a core is covered with the resist 4 (FIG. 1E). Then, with the residual resist 4 as an etching mask, the exposed area of the core layer 3a is removed by reactive ion etching (RIE), a core 3 is formed under the resist 4 (FIG. 1F), and the resist 4 is removed and the core 3 is exposed (FIG. 1G). Thereafter, cladding material is accumulated and hardened on the core 3 and the lower cladding layer 2, and an upper cladding layer 6 is formed. Thereby, an optical wave guide is obtained wherein a channel type core 3 is embedded between the lower cladding layer 2 and the upper cladding layer 6 (FIG. 1H).
According to the semiconductor process, the core 3 is formed into a rectangular or square cross sectional shape as shown in FIG. 2A along the full length of the core 3. Alternatively, as shown in FIG. 2B, by tapering both the sides by etching, the core is made into a trapezoidal shape (a mesa structure) wherein the bottom side (the side contacting the lower cladding layer 2) is longer than the top side.
In such an optical wave guide as described above, light coming from the incoming end of the core 3 into the core 3 is totally reflected by interfaces among the lower cladding layer 2 and the upper cladding layer 6 and the core 3 (for example, the upper and lower surfaces and left and right side surfaces of the core 3 in the case of a rectangular core, is transmitted through the inside of the core 3, and is ejected via a light outgoing end to the outside. Main causes of transmitting loss in light transmitted through the core 3 include loss arising at curved portions of the core 3 and loss arising at end surfaces of the core 3.
First, the loss at the curved portions of the core 3 is explained below. As shown in FIGS. 3 and 4, with respect to the loss arising at the curved portion in the curved core 3, when light comes from the straight portion of the core 3 into the external circumferential surface of the curved portion, the incident angle θ2 of light coming into the external circumferential surface of the core 3 becomes smaller than the critical angle of full reflection. Thus, light is not fully reflected to the inside of the core 3, and light is radiated from the core 3 to the upper cladding layer 6. Consequently, loss occurs.
FIG. 5 and FIG. 6 explain transmitting loss at the curved portion of the core 3 in viewpoint of wave optics. FIG. 5 shows the effective refractive index along the line A-A′ in the straight portion of the core 3 in FIG. 3 (the effective refractive index of a straight optical wave guide) and the effective refractive index along the line B-B′ in the curved portion (the effective refractive index of a curved optical wave guide).
In the straight optical wave guide, the distribution of the effective refractive index in the direction crossing the core 3 appears to be a symmetrical one to the center of the core 3, as shown by the dotted line in FIG. 5, while in the curved optical wave guide, as shown by the solid line in FIG. 5. The effective refractive index becomes high at the external circumferential side, and the effective refractive index becomes low at the internal circumferential side. For this reason, light being transmitted through the core 3 is confined in the core 3 at the portion of the straight optical wave guide. However, while it passes the portion of the curved optical wave guide, the light is radiated and expanded into the upper cladding layer 6 at the external circumferential side where the refractive index gets high. As a result, loss occurs. Also, at the portion of the curved optical wave guide, where the electric field distribution of wave guide mode is distorted, irregularity of the electric field distribution occurs at the inlet of the curved optical wave guide portion and light is radiated to the outside of the core 3. As a result, loss occurs.
In order to restrict the transmitting loss at the curved portion of the core 3, it may be effective to make the radius of curvature R of the curved portion large so that the incident angle to the external circumferential surface of the core 3 should become larger than the critical angle. However, making the radius of curvature R large requires lengthening of the curved portion of the core 3 so as to obtain a necessary curve angle. As a result, an optical wave guide becomes long and large. Therefore, in prior art optical wave guides, the reduction of transmitting loss in the curved portion and the compact size of an optical wave guide have been a tradeoff.
Next, the joint loss arising at the end surfaces of the core 3 is described below. The loss arising at the end surfaces of the core 3 arises at optical connections at the light incident end or the light outgoing end of the core 3, other optical element and the like. In order to reduce such joint loss, in a single mode core 3, it is preferred that the shapes of the light incident end and the light outgoing end are close to the end surface shape of an optical fiber or the like to be connected to the core 3.
Next, an optical switch according to the prior art as an application of an optical wave guide is explained below. FIGS. 8A, 8B, and 8C are the cross sectional views showing cross sections at C1-C1′, C2-C2′, and C3-C3′ in FIG. 7 respectively. In an optical switch, a Y-branched core 3 is formed on a substrate 1, and two units of a heater 7 are arranged so as to pinch the top of the branch portion of the core 3 at the upper surface of an upper cladding layer 6 covering the core 3. The core 3 appears to be a rectangular at any cross section in FIGS. 8A, 8B, and 8C, or appears a narrow trapezoid at the heater sides.
In the above structure, when one of the heaters 7 is turned on and thereby heat is generated, the temperature goes up at the heated heater 7 side. As a result, the refractive indexes of the core 3 and the upper cladding layer 6 at the heat generating side become small by thermal optical (TO) effect, and light transmitted through the non-branched portion of the core 3 is transmitted to the branch portion at non-heat generating side, and the light is hardly transmitted to the heat generating side. Thus, by switching over the heater 7 to be heated, it is possible to switch the route of the light transmission (branch direction).
In such an optical switch, functionality to completely switch light routes just as in an electric switch is required, and there is a demand for an optical switch having a high extinction ratio. In such an optical switch, so as to obtain a high extinction ratio, it is necessary to make the branch angle of the core 3 small, or to make the refractive index difference given by heaters large.
However, when the branch angle is made small, if the interval between the branch portions of the core 3 at light outgoing end is tried to be made sufficiently large so as to connect an optical fiber thereto, the core length becomes long until the branch portions of the core 3 get apart sufficiently. Consequently, the size of an optical switch will become long and large, which has been a problem in the prior art.
Further, when the refractive index difference given by heaters is made large, the amount of heat generated by the heaters is required to be increased. As a consequence, electric consumption of an optical switch will become higher, which has been another problem in the prior art. Moreover, when the refractive index change is large, wherein an abrupt change of refractive index occurs in wave guide direction, multi-mode occurs in a single mode core. As a result, excessive loss will occur. This leads to an undesired reduction of extinction ratio, which has been still another problem in the prior art.